Field of the Invention
The present invention relates to an organic light emitting device. More specifically, the present invention relates to a white organic light emitting device in which the efficiency of a blue fluorescent stack is improved to be similar to that of a phosphorescent stack.
Discussion of the Related Art
The recent trend toward information-dependent age has brought about rapid development in display fields in which electrical information signals are visually displayed. In this regard, a variety of flat display devices having superior properties such as slimness, low weight and low power consumption have been developed and are actively used as substitutes for conventional cathode ray tubes (CRTs).
Specific examples of flat display devices include liquid crystal display devices (LCDs), plasma display panel devices (PDPs), field emission display devices (FEDs), organic light emitting devices (OLEDs) and the like.
Among these, organic light emitting devices that do not require an additional light source, realize a compact design and render clear color are considered to be competitive applications.
Such organic light emitting devices require formation of organic light emitting layers.
An organic light emitting device in which a stack structure including organic light emitting layers having different colors is laminated without patterning the organic light emitting layers at respective pixels in order to render white light was suggested.
That is, in white organic light emitting devices, light emitting diodes are formed by depositing layers between an anode and a cathode without using a mask, and organic films including organic light emitting layers are sequentially deposited under vacuum using different materials.
White organic light emitting devices are utilized in various applications including thin light sources, backlights of liquid crystal display, or full-color displays using color filters.
Meanwhile, each stack of conventional white organic light emitting devices includes a hole transport layer, a light emitting layer and an electron transport layer.
In this regard, when a white organic light emitting device having a laminate structure including a fluorescent stack and a phosphorescent stack is formed, the fluorescent stack exhibits a relatively low efficiency. As a result, great difference in color upon operation at low current occurs.
These conventional white organic light emitting devices have the following problems.
When the white organic light emitting device having a laminate structure including a fluorescent stack and a phosphorescent stack is formed, the fluorescent stack exhibits relatively low efficiency. In particular, color of fluorescence emitted from the fluorescent stack during operation at a low current has a considerably low intensity, while the color of phosphorescence emitted from the phosphorescent stack has an intensity of a predetermined level or more. That is, the difference in color therebetween is great.
The reason is as follows.
That is, triplet excitons of the light emitting layer are diffused from the fluorescent stack into the hole transport layer of the fluorescent stack due to low triplet energy level. The reason for this is that diffusion distance of excitons from the fluorescent stack is longer than the thickness of a commonly used blocking layer. Such exciton diffusion occurring at a low current at which sufficient excitons are not formed greatly deteriorates efficiency of a blue fluorescent stack and decreases efficiency.
For this reason, a screen displayed by a display device to which the white organic light emitting device is applied renders warm white in color that appears redder or yellower than a normal state. At this time, a viewer recognizes deterioration in image quality.